1. Field of the Invention
This invention pertains to the art of rotary rock drilling and, more spcifically, to dynamic grease seals employed to protect bearings supporting rotating conical rock cutters of sealed bearing rotary cone rock bits.
As a well is drilled progressively deeper into the rock mantle of the earth, it becomes progressively more difficult and time consuming to withdraw the drill string, replace a faulty rock bit and return the new bit to the borehole.
One type of drill bit failure requiring such a replacement occurs when wear in a bearing grows to a point where eccentric excursions of the rotary cutter upon its supporting journal exceed the capacity of the resilient seal to compensate for the eccentricity. At this point, drilling fluid, laden with destructive abrasives, passes the protective seal and destroys the bearings.
The capacity of the seal to compensate for eccentricity might have been greater had the O-ring been initially more tightly packed or "squeezed" in its housing gland. However, it has been observed with higher squeeze pressures imposed upon the O-ring, seal life is shortened and caused to fail through bunching, snaking or twisting.
It is well known that friction bearings generally operate most of their useful life with little measurable wear. Eventually, however, wear mechanisms give rise to asperities on the bearing surface. Small asperities wear smooth again only to reappear nearly through the transfer of small amounts of metal from surface to surface as rotation progresses. The asperities grow in area and the rate of growth increases with operating time, until ultimately the bearing fails. It is noted that this process produces heat, at a rate increasing as the area and degree of bearing damage increases. A degenerating bearing thus undergoes a temperature rise as cooling forces are both constant and limited.
2. Description of the Prior Art
U.S. Pat. No. 3,397,928 teaches the use of a radially loaded O-ring to seal the bearing space of a rotating cone rock bit. Novelty is claimed in the use of a percentage rate of squeeze in excess of the published limits recommended by O-ring manufacturers as permitting useful life expectancy in service. The patent teaches a minimum of 10.0 percent with an ideal squeeze of 15.0 percent on the O-ring or seal packing at rock bit assembly. As noted heretofore, excessive O-ring squeeze results in shortened life through bunching, snaking and twisting.